Natural gas hydrate (or “gas hydrate”, for short), is an ice-like, non-stoichiometric clathrate compound, which is formed by the combination of water and hydrocarbons having low molecular weights in the natural gas under low temperature and high pressure. Naturally occurring gas hydrate is mainly methane hydrate, which mostly occurs under the seafloor and has a few advantages such as its large quantities, wide distribution, shallow occurrence, high energy density, and residue- and pollution-free burning. One unit volume of methane hydrate produces 150 to 180 unit volumes of methane gas after dissociation. It is estimated that, natural gas hydrate represents 53% of the global organic carbon reservoir, two times of the total amount of the three fossil fuels (coal, oil and natural gas). Thus, natural gas hydrate has been considered as an ideal clean alternative energy in the 21st century.
Natural gas hydrate, occurring in solid form in loose sediments of muddy sea bottom, will undergo a phase transition during its exploitation, and thus huge difficulties in gas hydrate exploitation exist compared with oil and natural gas exploitations. Depending on where the gas hydrate dissociates, there are two kinds of gas hydrate exploitation, underground dissociation exploitation and above-ground dissociation exploitation.
The above-ground dissociation exploitation is mainly applied to shallow and non-diagenetic hydrate reservoirs. Chinese patent CN1294648A discloses a method in which high pressure air is introduced to the natural gas hydrate reservoir and the solid gas hydrate is carried over in a flow to the ocean surface. Chinese patent CN1587642A discloses a process based on the onshore mining method, in which solid gas hydrate is extracted by underwater automatic excavators, and then recovered by silt separation and gas hydrate dissociation. Chinese patent CN105587303A discloses a green method and device for exploitation of shallow and non-diagenetic gas hydrate reservoirs at seafloor. CN105064959A discloses a green method for exploitation of seafloor non-diagenetic gas hydrate reservoirs, in which solid gas hydrate is extracted through submarine mining, and after a secondary crushing the solid particles of gas hydrate is mixed with seawater in a confined room to discompose it into natural gas and water utilizing the heat of the seawater from the ocean surface, and then lift to the ocean surface by airlift effect. All the above methods for above-ground dissociation exploitation have problems such as their limited applicability, high technical demand on underwater automatic mining machines, difficult implementation, and huge damages to seafloor geological structure which will cause well collapses or landslides.
Most researches and reports focus on underground dissociation exploitation mainly based on exploitation techniques of oil and natural gas, in which a wellbore is constructed in the seafloor stratum, and specific methods will be adopted to change the thermodynamic conditions, such as temperature and pressure, to precipitate an in-situ dissociation of the natural gas hydrate into water and natural gas. The water and the natural gas are collected and separated, and then delivered to the ocean surface through a marine riser. Methods for underground dissociation exploitation include thermal stimulation, depressurization, and chemical method. At present, most researches of underground dissociation exploitation focus on how to dissociate the gas hydrate in situ in the stratum through an economical, safe and efficient method. In contrast, fewer researches focus on how to deliver the mixture of gas, water and sand from the well bottom to the platform at the ocean surface. In the first producing test of marine natural gas hydrate at Naikai Through, Japan in 2013, electric submersible pumps were adopted to pump the gas-water mixture from the well bottom through the exploitation well to a gas-liquid separator, and then the separated gas phase and water phase were delivered to the ocean surface separately through two marine risers. In the producing test of natural gas hydrate by China Geological Survey at Shenhu Area of South China Sea in 2017, high power electric submersible pumps were adopted to deliver the geological fluid of gas-water mixture in the hydrate layer through exploitation well and marine riser, and then the mixture were dissociated into methane gas and water. These methods of recovering the gas by adopting electric submersible pumps have a high cost, due to the high energy consumption and short operation life of electric submersible pumps. Thus, there is a need to develop an economical and efficient technology for delivering the gas in natural gas hydrate exploitation, which can be applied in exploiting marine natural gas hydrate resource.